1. Field of the Invention
The present invention relates generally to wireless communication systems, and in particular relates to transponders and transponder systems and methods used in optical-fiber-based wireless picocellular systems for radio-over-fiber (RoF) communication.
2. Technical Background
Wireless communication is rapidly growing, with ever-increasing demands for high-speed mobile data communication. As an example, so-called “wireless fidelity” or “WiFi” systems and wireless local area networks (WLANs) are being deployed in many different types of areas (coffee shops, airports, libraries, etc.). Wireless communication systems communicate with wireless devices called “clients,” which must reside within the wireless range or “cell coverage area” in order to communicate with the access point device.
One approach to deploying a wireless communication system involves the use of “picocells,” which are radio-frequency (RF) coverage areas having a radius in the range from about a few meters up to about 20 meters. Because a picocell covers a small area, there are typically only a few users (clients) per picocell. Picocells also allow for selective wireless coverage in small regions that otherwise would have poor signal strength when covered by larger cells created by conventional base stations.
In conventional wireless systems, picocells are created by and centered on a wireless access point device connected to a head-end controller. The wireless access point device includes digital information processing electronics, an RF transmitter/receiver, and an antenna operably connected to the RF transmitter/receiver. The size of a given picocell is determined by the amount of RF power transmitted by the access point device, the receiver sensitivity, antenna gain and the RF environment, as well as by the RF transmitter/receiver sensitivity of the wireless client device. Client devices usually have a fixed RF receiver sensitivity, so that the above-mentioned properties of the access point device mainly determine the picocell size. Combining a number of access point devices connected to the head-end controller creates an array of picocells that cover an area called a “picocellular coverage area.” A closely packed picocellular array provides high per-user data-throughput over the picocellular coverage area.
Prior art wireless systems and networks are wire-based signal distribution systems where the access point devices are treated as separate processing units linked to a central location. This makes the wireless system/network relatively complex and difficult to scale, particularly when many picocells need to cover a large region. Further, the digital information processing performed at the access point devices requires that these devices be activated and controlled by the head-end controller, which further complicates the distribution and use of numerous access point devices to produce a large picocellular coverage area.
Radio-over-Fiber (RoF) wireless picocellular systems utilized optical fibers to transmit the RF signals to RoF transponders that convert the RF optical signals to electrical RF signals and then to wireless electromagnetic (EM) signals, and vice versa. Unlike conventional wireless system access points, the RoF transponders generally do not require any signal processing capability, thereby simplifying the distribution of the RoF transponders to produce a large picocellular coverage area.
While RoF wireless picocellular systems are generally robust, there are some shortcomings. One shortcoming relates to the relative difficulty in manufacturing and deploying an optical fiber cable having a linear array of transponders. Each transponder needs to be optically coupled to an uplink optical fiber and a downlink optical fiber as well as to an electrical power line, usually via a “tether cable.” This involves the tedious and time-consuming process of accessing the uplink and downlink optical fibers and the electrical power line in the cable, splicing the optical fibers and electrical power line, and then connecting them to the transponder. Another shortcoming of the linear array approach for distributing transponders is that the approach is not readily scalable once the system is deployed. This makes it difficult to quickly and inexpensively change the picocell coverage area to accommodate the changing needs or geometry of the particular wireless environment.